In a double-enveloped lamp having an inner light-source capsule, there is a small probability that the capsule will burst. If such an event occurs, the hot fragments of glass or shards and other capsule parts emanating from the burst capsule will be forcibly propelled against the outer envelope. If the outer envelope also shatters, there may be a safety hazard to persons or property in the immediate surroundings. In such a case, a "containment failure" of the lamp or outer envelope has occurred, since the outer envelope has failed to contain internal lamp parts within the lamp.
The cause of a lamp containment failure is unknown and unpredictable. There is no known way to eliminate the possibility of such a failure. Although its occurrence is rare, the consequences of a containment failure may be serious. Therefore, protective measures must be taken.
Lamp manufacturers regularly notify users of the possibility of a containment failure by means of warnings on packages and in descriptive materials. Suggested precautions are often included in specifications and operating instructions. One way to avoid the safety hazard is to operate the lamp within a protective fixture itself capable of containing such a failure. This method is more acceptable in commercial usage than in the consumer market, but it has disadvantages in either case. A protective fixture generally incurs additional cost, particularly if an existing fixture has to be modified or replaced. A protective lens reduces the light output of the lamp somewhat. It may be more difficult and expensive to replace a lamp in a protective fixture, and replacement of a lamp with a shattered outer envelope is itself a safety concern. There may be other technical or aesthetic drawbacks.
A preferred solution to the containment failure problem is clearly a lamp capable of self containment. To this end, there are several known techniques. One technique is to make the outer envelope stronger so that it will contain. In U.S. Pat. No. 4,598,225, issued July 1, 1986, to Gagnon, there is shown an outer envelope having a thick outer wall in combination with a light-source capsule with a thin inner wall. Another technique is that of shielding the outer envelope from the effects of a burst capsule. In U.S. Pat. No. 4,580,989, issued Apr. 8, 1986, to Fohl et al., a light-transmissive enclosure within the outer envelope surrounds the light-source capsule and shields the outer envelope. Also see Bechard et al., U.S. Pat. No. 4,281,274, issued July 28, 1981. Yet another technique is to reinforce the outer envelope or shield. In some cases, a light-transmissive coating may be applied to the outside surface of the outer envelope. See Ser. No. 088,500, filed Aug. 17, 1987 a continuation of Ser. No. 650,938, filed Sept. 17, 1984. In other cases, the shield may be reinforced by a wire mesh surrounding the outside surface thereof. For example, see Ser. No. 873,292, filed June 5, 1986, being a continuation of Ser. No. 744,645, filed June 13, 1985, which is a continuation of Ser. No. 422,312, filed Sept. 23, 1982.
These techniques are effective, particularly with lamps of lower wattages. However, as wattage increases, e.g., one hundred and seventy-five watts and higher, the energy released by a burst capsule is proportionately greater. The mentioned techniques cannot be relied on for certain containment, and improved techniques are still being sought.
It would be an advancement of the art if a lamp structure were provided wherein the outer envelope would reliably contain a burst of the inner light-source capsule even where the lamp wattage may be one-hundred and seventy-five watts or higher.